1. Field of the Invention
The present invention relates to a variable geometry turbine and has particular, but not exclusive, application to variable geometry turbochargers.
2. Description of the Related Art
Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.
The turbine stage of a conventional turbocharger comprises: a turbine housing defining a turbine chamber within which the turbine wheel is mounted; an annular inlet passage defined in the housing between facing radially extending walls arranged around the turbine chamber; an inlet arranged around the inlet passage; and an outlet passage extending from the turbine chamber. The passages and chamber communicate such that pressurised exhaust gas admitted to the inlet flows through the inlet passage to the outlet passage via the turbine chamber and rotates the turbine wheel. It is known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passage so as to deflect gas flowing through the inlet passage towards the direction of rotation of the turbine wheel.
Turbines of this kind may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passage can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied in line with varying engine demands.
In one known type of variable geometry turbine, an axially moveable wall member defines one wall of the inlet passage. The position of the movable wall member relative to a fixed facing wall of the inlet passage is adjustable to control the axial width of the inlet passage. Thus, for example, as exhaust gas flow through the turbine decreases, the inlet passage width may be decreased to maintain the gas velocity and optimise turbine output.
The axially movable wall member may be a “nozzle ring” that is provided with vanes that extend into the inlet passage and through orifices provided in a shroud plate defining the fixed facing wall of the inlet passage, the orifices being designed to accommodate movement of the nozzle ring relative to the shroud. Typically the nozzle ring may comprise a radially extending wall (defining one wall of the inlet passage) and radially inner and outer axially extending walls or flanges that extend into an annular cavity behind the radial face of the nozzle ring. The cavity is formed in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing) and accommodates axial movement of the nozzle ring. The flanges may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. In one common arrangement the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator, which axially displaces the rods.
In an alternative type of variable geometry turbocharger, the nozzle ring is fixed and has vanes that extend from a fixed wall through orifices provided in a moving shroud plate.
Actuators for moving the nozzle ring or movable shroud plate can take a variety of forms, including pneumatic, hydraulic and electric and can be linked to the nozzle ring or shroud plate in a variety of ways. The actuator will generally adjust the position of the nozzle ring or movable shroud plate under the control of an engine control unit (ECU) in order to modify the airflow through the turbine to meet performance requirements.
One example of a variable geometry turbocharger is disclosed in EP 0654587, which discloses a nozzle ring that is additionally provided with pressure balancing apertures through its radial wall. The pressure balancing apertures ensure that pressure within the nozzle ring cavity behind the nozzle ring is substantially equal to, but always slightly less than, the pressure applied to the nozzle ring face by gas flow through the inlet passage. This ensures that there is only a small unidirectional force on the nozzle ring which aids accurate adjustment of the nozzle ring position, particularly when the nozzle ring is moved close to the opposing wall of the inlet to reduce the inlet passage towards its minimum width.
In addition to the conventional control of a variable geometry turbocharger in an engine fired mode (in which fuel is supplied to the engine for combustion) to optimise gas flow, it is also known to take advantage of the facility to minimise the turbocharger inlet area to provide an engine braking function in an engine braking mode (in which no fuel is supplied for combustion) in which the inlet passage is reduced to smaller areas compared to those in a normal engine fired mode operating range.
Engine brake systems of various forms are widely fitted to vehicle engine systems, in particular to compression ignition engines (diesel engines) used to power large vehicles such as trucks. The engine brake systems may be employed to enhance the effect of the conventional friction brakes acting on the vehicle wheels or, in some circumstances, may be used independently of the normal friction braking system, to control, for example, the downhill speed of a vehicle. With some engine brake systems, the brake is set to activate automatically when the engine throttle is closed (i.e. when the driver lifts his foot from the throttle pedal), and in others the engine brake may require manual activation by the driver, such as depression of a separate brake pedal.
In one form of conventional engine brake system an exhaust valve in the exhaust line is controlled to block partially the engine exhaust when braking is required. This produces an engine braking torque by generating a high backpressure that retards the engine by serving to increase the work done on the engine piston during the exhaust stroke. This braking effect is transmitted to the vehicle wheels through the vehicle drive chain. U.S. Pat. No. 4,526,004 discloses such an engine braking system for a turbocharged engine in which the exhaust valve is provided in the turbine housing of a fixed geometry turbocharger.
With a variable geometry turbine, it is not necessary to provide a separate exhaust valve. Rather, the turbine inlet passage may simply be “closed” to a minimum flow area when braking is required. The level of braking may be modulated by control of the inlet passage size by appropriate control of the axial position of the nozzle ring or movable shroud plate. In a “fully closed” position in an engine braking mode the nozzle ring or movable shroud plate may in some cases abut the facing wall of the inlet passage. In some exhaust brake systems known as decompression brake systems, an in-cylinder decompression valve arrangement is controlled to release compressed air from the engine cylinder into the exhaust system to release work done by the compression process. In such systems closure of the turbine inlet both increases back pressure and provides boost pressure to maximise compression work.
A variable geometry turbocharger can also be operated in an engine fired mode so as to close the inlet passage to a minimum width less than the smallest width appropriate for normal engine operating conditions in order to control exhaust gas temperature. The basic principle of operation in such an “exhaust gas heating mode” is to reduce the amount of airflow through the engine for a given fuel supply level (whilst maintaining sufficient airflow for combustion) in order to increase the exhaust gas temperature. This has particular application where a catalytic exhaust after-treatment system is present. In such a system performance is directly related to the temperature of the exhaust gas that passes through it.
To achieve a desirable performance the exhaust gas temperature must be above a threshold temperature (typically lying in a range of about 250° C. to 370° C.) under all engine operating conditions and ambient conditions. Operation of the exhaust gas after-treatment system below the threshold temperature range will cause the system to build up undesirable accumulations which must be burnt off in a regeneration cycle to allow the system to return to designed performance levels. In addition, prolonged operation of the exhaust gas after-treatment system below the threshold temperature without regeneration will disable the system and cause the engine to become non-compliant with government exhaust emission regulations.
For the majority of the operating range of, for example, a diesel engine, the exhaust gas temperature will generally be above the required threshold temperature. However, in some conditions, such as light load conditions and/or cold ambient temperature conditions, the exhaust gas temperature can often fall below the threshold temperature. In such conditions the turbocharger can in principle be operated in the exhaust gas heating mode to reduce the turbine inlet passage width with the aim of restricting airflow thereby reducing the airflow cooling effect and increasing exhaust gas temperature.
For both engine braking and exhaust gas heating, is important to allow some exhaust gas flow through the turbine of the turbocharger. If the exhaust from an engine is restricted to too great an extent, this can lead to excessive heat generation in the engine cylinders, failure of exhaust valves, and the like. There must therefore be provision for at least a minimum leakage flow through the turbine when the nozzle ring or movable shroud plate is in a fully closed position in an engine braking mode.
However, due to their high efficiency modern variable geometry turbochargers can generate such high boost pressures even at small inlet widths that their use in an engine braking mode can be problematic as cylinder pressures can approach, or exceed, acceptable limits unless countermeasures are taken (or braking efficiency is sacrificed). This can be a particular problem with engine brake systems including a decompression braking arrangement. Similarly, in relation to exhaust gas heating, the high boost pressures achieved at small inlet widths can actually increase the airflow to the engine, offsetting the effect of the restriction and thus reducing the desired heating effect.
These problems have been addressed up to a point by documents such as EP145434 and US2005/0060999A1, which teach the use of bypass passages which open when a movable nozzle ring is closed, increasing the flow rate of exhaust through the turbine and limiting the exhaust pressure (and heat build-up) at the engine. However, such bypass passages can be prone to clogging due to build-up of soot.